Within the technical field of the present invention, a sinusoidal alternating voltage with a frequency between 20 and 50 Hz and with a relatively high amplitude up to 250 V (peak to peak) is used as a ring signal.
Generally, there are two different solutions known in the art to generate such a ring signal. Within the first solution, a central generator is connected with one lead, normally the RING lead, by means of a relay. The other lead, normally the TIP lead, is connected to ground. The electronic subscriber line interface circuit (SLIC) has to be disconnected from the line because of the high voltage of 250 V. This solution is known as unbalanced solution, because only one lead is used.
Within the second solution, it is utilized that the driver circuit of the subscriber line interface circuit has the ability to supply a signal with sufficient amplitude to the TIP and RING lead. In this case, the TIP and RING leads are driven differentially, and balanced respectively. Advantageously, the balanced solution comprises the half voltage swing compared to the unbalanced solution. However, there is also a problem remaining. Due to the high power consumption of the driver circuit, there is a high power loss.
A way to reduce the power loss is described in the patent application EP 0903926 A1. The aim of this patent application is that the power loss of the power amplifier which amplifies a low amplitude ring voltage is reduced in that the operating voltage respectively supply voltage of the power amplifier is not constant, but that it is controlled. This takes place through a switching controller so that in time segments the operating voltage respectively supply voltage is only insignificantly greater than the ring voltage. A disadvantage here is that two different supply voltages are needed for providing an alternating ring signal. Two different supply voltages mean a higher effort in place and a higher complexity in the circuitry. For example, FIG. 1 shows a V-t-diagram illustrating the balanced generation of the ring signal by means of two different supply voltages. The ring signal VTR is defined by the difference between the TIP potential VT and the RING potential VR. FIG. 1 shows that for the generation of the TIP potential VT, a negative supply voltage V− is needed, whereas for the generation of the RING potential VR, the positive supply voltage V+ is needed. The TIP potential VT and the RING potential VR are phase-delayed at 180°.